This invention relates to high-temperature materials. More particularly, this invention relates to metal alloys and articles for high-temperature service, and to methods for making such alloys and articles.
The remarkable strength of many superalloys is primarily attributable to the presence of a controlled dispersion of one or more hard precipitate phases within a comparatively more ductile matrix phase. For instance, many nickel-based superalloys are primarily strengthened by an intermetallic compound known as “gamma-prime.” In general, articles formed from these alloys are processed to achieve a target grain size, then heat treated to achieve a dispersion of gamma-prime precipitates having desired size and morphology to provide the balance of properties specified for the material. This heat treatment typically involves at least three phases. First, the material is given a “solutionizing” heat treatment above the gamma-prime solvus temperature to dissolve any gamma-prime that may have formed during solidification and/or other prior processes (referred to as “primary gamma-prime”). Then the material is cooled either very rapidly, or in a controlled manner, to allow precipitation of gamma-prime of a desired size and shape. Finally, if needed, the material is subsequently given another heat treatment, called an “aging” treatment, at a temperature below the gamma-prime solvus, to allow the gamma-prime to precipitate to the degree specified for the given application. Multiple cooling and aging steps may be used to effect precipitation of gamma-prime having various sizes and shapes. The material is then processed to final dimensions via various known forming and machining methods.
The grain size of the alloy is another micro structural feature that plays a measurable role in determining some properties of the material. As the material is heated to high temperatures, the grains in the material are energetically favored to grow. However, in some applications, the grain size is desired to be quite small, and thus controlling grain size during thermal processing is an important consideration. In alloys where gamma-prime is the primary precipitate phase in the microstructure, maintaining a desirable grain size can be problematic when gamma-prime is completely or nearly completely dissolved during the “solutionizing” heat treatment, because gamma-prime is the primary grain size controlling phase in the material due to its ability to pin grain boundaries to inhibit growth. With no gamma-prime in the microstructure, and at elevated temperature, grain growth can occur because there are substantially no other phases present in the microstructure to prevent growth. To address this issue, heat treatment processes have been developed wherein a certain amount of primary gamma-prime is allowed to remain undissolved during heat treatment, leaving the primary gamma-prime to perform a grain boundary pinning function during heat treatment. As a result, the gamma-prime distribution in the processed part will include not only the fine dispersion of gamma-prime generated during the aging step(s), but also a population of typically coarser primary gamma-prime that is generally not as effective in contributing strength to the material. On the other hand, processes that dissolve substantially all of the primary gamma prime may result in an overall finer dispersion of gamma prime, but generally result in material having a coarser grain size than is desirable for certain applications.
Therefore, there remains a need in the art for materials and methods that allow for the combination of fine grain size with fine dispersions of gamma-prime phase to optimize the properties of articles used in high temperature applications, such as turbomachinery components.